A method for controlling a magnetic powder clutch of a transmission installed between an engine and drive wheels has been disclosed in Japanese Patent Application No. Sho 60-171665. In this art, when a vehicle is starting, the transmitting torque of the magnetic powder clutch is developed from 0 N.multidot.m in the idling state to be equal to an engine torque at a target engagement revolution speed. Here, the engagement revolution speed is obtained when the revolution speed of the input shaft and the output shaft of the clutch is substantially the same. The transmitting torque TCL of the magnetic powder clutch is controlled based on the following equation (1). EQU TCL=Te+k.multidot.(Ne-N*) (1),
where
Te: engine torque, PA1 Ne: engine revolution speed PA1 k: feedback gain, and PA1 N*: target engagement revolution speed. PA1 TCL: a required transmitting torque, and PA1 Te: an estimated engine torque. PA1 In : inertia moment of engine, PA1 Ne : change rate in the engine revolution speed, PA1 Te : engine torque, and PA1 TCL : transmitting torque. PA1 TCLO: a preset transmitting torque, and PA1 TCLB: decreased transmitting torque.
The transmitting torque TCL is thus calculated based on the current engine torque Te and the current engine revolution speed Ne. Then, an energizing voltage VCL responsive to the transmitting torque TCL is determined in reference to the characteristic curve of the magnetic powder clutch shown in FIG. 18. By applying the voltage VCL to the magnetic powder clutch, the transmitting torque TCL becomes equal to the engine torque Te at the target engagement revolution speed N*. In the above control method, however, the characteristic at the starting of the vehicle changes since the efficiency of the transmitting torque of the clutch gradually changes with the lapse of time. To cope with this, an improved version of this method has been developed in Japanese Published Unexamined Patent Application No. Sho 60-164025 in that the change in the efficiency of the transmitting torque of the magnetic powder clutch is detected based on a stall revolution speed of the clutch, i.e., a revolution speed at which the engine revolution speed is saturated under the condition of zero vehicle speed and wide-open throttle. Thus, the change in the transient characteristic of the magnetic powder clutch has been controlled.
However, when the transmitting torque TCL of the magnetic powder clutch is controlled to be equal to the engine torque at the target engagement revolution speed N*, a problem set forth sometimes occurs. FIG. 19 shows the relationship between the transmitting torque TCL and the energizing voltage VCL, the solid line indicates a designated curve. If the designated curve shifts in the direction of YA or YB with the lapse of time, the transmitting torque TCL does not necessarily become equal to the engine torque at the target engagement revolution speed N*. When the vehicle is starting, a required transmitting torque TCL that the magnetic powder clutch should transmit is calculated as follows in reference to equation (1). EQU TCL=Te+k.multidot.(Ne-N*) (2)
where
Accordingly, if the actual transmittign torque TCL of the magnetic powder clutch is equal to the required transmitting torque TCL, and also the actual engine torque Te is equal to the estimated engine torque Te, the control for the magnetic powder clutch at the starting of the vehicle is accurately performed according to equation (1) so that the clutch is engaged at the target engagement revolution speed N*.
Generally, the relationship of equation (3) exists in the engine and the magnetic powder clutch system.
In.multidot.Ne=Te-TCL (3)
In reference to equations (2) and (3), when TCL=TCL, the engine revolution speed Ne changes in relation to time t based on the following equation (4). EQU In.multidot.Ne-k.multidot.(Ne-N*) (4)
From equation (4), the engine revolution speed Ne is calculated as shown in the equation (5). EQU Ne=Nidle.multidot.exp(-k.multidot.t/In)+{1-exp(-k.multidot.t/In)}.N*(5)
When t=0, Ne becomes equal to Nidle, As the time t approaches infinity (t.fwdarw..infin.), the engine revolution speed gets closer to the target engagement revolution speed N*. On the other hand if TCL.noteq.TCL, the following equation (6) is obtained in reference to the equations (2) and (3). ##EQU1##
Here, the term (Te-Te) is involved in the equation. The engagement revolution speed, therefore, does not coincide with the target engagement revolution speed N*.
The change in the clutch characteristic sometimes brings about problems such as shock or delay in the starting of the vehicle, or deterioration of the accuracy in disengagement. Though it is possible to detect the change in the magnetic powder clutch characteristic from its stall revolution speed when the vehicle stops, a further problem is left. Namely, the stall revolution speed of the magnetic powder clutch can not be detected under the usual driving condition. To make it possible, a driver must apply the hand-brake and depress the accelerator to make the throttle wide-open. As a result, hard noise is caused and the durability of the magnetic powder clutch deteriorates.